How can any Christian say that they are certain of salvation?

I have grave doubts about the process by which belief becomes certainty. When first considering a topic I had never before thought about, strong beliefs sometimes spring forward. I don't know where they come from. For example, when I watch a hockey game between two unfamiliar teams, by the end of it I usually favor one of them to win. I couldn't tell you why. The belief that one team is preferred simply appears, fully formed. This certainly doesn't arise from any rational inference; it just happens.

Sometimes beliefs subside in the face of evidence. After the belief is already held, my defensive attempts at justification, logic, rationalizing, etc. might fail. If through some long effort I find myself unable to justify the belief, then the belief itself will fade. At the moment the belief goes away, I can say I am convinced or persuaded of its falsity. But these are just words that mean the belief went away. If I'm not convinced yet, I can't simply decide to be convinced; I have to be convinced. There must be some threshold quantity of evidence or argument at which persuasion occurs, but I am not aware of setting or controlling this threshold in any way.

Now, whatever their source, all of these beliefs are convincing to me, by definition. Some beliefs might be more easily dislodged than others, but while they are still held, I believe them. If I didn't, they wouldn't be beliefs. Some beliefs might be so intensely held that no possible persuasion could ever dislodge them. But I cannot know this with certainty, because I cannot (in a finite lifetime) experience all possible persuasion. I can know that a belief was not held with certainty, only after it ceases to be a belief.

So to say that you are certain of salvation means that no amount of evidence could ever persuade you that you were not saved. However, many other people who have believed themselves certain of salvation have then gone on to become atheists, and stopped believing they were saved (or that salvation is even a thing). Therefore, their belief in salvation was not held with certainty, as proven by the fact that it was in fact changed by persuasion.

So how can anyone be certain of salvation? How can you be sure you won't someday be persuaded to be an atheist?